1. Field of the Invention
The present invention relates to the use of polymeric polyamines as exfoliating agents for producing random nanosilicate plates from layered silicate clays, and particularly to a method for producing nanometer scale silicate plates by directly exfoliating inorganic layered silicate clays with multiple functional quaternary salts, which are prepared specifically from amine-terminating Mannich oligomers (AMO) or amine-terminating epoxy oligomers (AEO). The invention also relates to a method involving ionic exchange between layered silicates and organic quaternary amines, aqueous sodium hydroxide extraction process, recycling the exfoliating agents and finally the isolation of random silicate plates.
2. Description of the Related Art
Currently, inorganic/organic polymer composite materials in a nanometer scale regime (approximately 1˜100 nanometer scale) are one of the most important materials, and thus have been widely investigated and developed. Such nanocomposite materials have two different phases including inorganic and organic components, wherein at least one phase is dispersed under a nanoscale regime in a homogeneous manner. Accordingly, the compatibility in the nanoscale mixing between two distinct phases, for example, inorganic clay and organic polymer, is the essential factor for the nanocomposite's physical and mechanical performance. In general, the clay/polymer hybrid materials can be classified into two categories of composites, the intercalated and the exfoliated, on the basis of the clay dispersion in polymer matrix. In an intercalated structure, the silicate plates still maintain their layered structure but with the addition of organic intercalants anchored in the gallery. In the exfoliated form, each individual silicate plate is randomly dispersed in the polymer matrix. Dramatically improved physical properties are often obtained for the exfoliated structure, as demonstrated by the first commercialized Nylon6/montmorillonite nanocomposites. In such a system, the silicate plates (about 100×100×1 nm in dimension) are well-entangled with polymer strands through van der Waals forces and evenly distributed in the polymer matrix. Only a low percentage of exfoliated silicate plates is required to enormously improve the mechanical and physical properties of the blended material.
Since the naturally occurring layered silicate clays are hydrophilic, the dispersion of clay in polymer matrix on a nanometer scale is a general problem. The process requires an enlargement of the clay interlayer distances by means of organic quaternary ammonium incorporation, and thus monomers can enter into the clay interface through ionic exchange reactions. The monomers can then be polymerized within the interlayer space to obtain an exfoliated inorganic/organic polymer nanocomposite material. In principle, the enlarged distance is preferred to be wide enough for monomer or polymer molecules to enter. After exfoliation, the layered structure is randomized into irregular shapes and the silicate plates have different directions without any crystalline form. The random silicate plates are therefore dispersed in organic polymers as nanocomposites.
Conventional intercalating agents such as 12-aminolauric acid, hexadecylamine, fatty amine, bis(2-hydroxyethyl) methyl tallow alkyl amine and stearylamine, usually have low molecular weights and can be converted into the corresponding ammonium salts such as quaternary ammonium chloride salt. Through ionic exchange reactions, the counter ions in interlayer spaces of the clay can be ionically exchanged and hence the interlayer distance expanded to a certain degree.
Referring to the research of T. J. Pinnavaia (Michigan State University), intercalating agent CH3(CH2)n—NH3+ is provided to exchange with metal ion salts within the layer-structured montmorillonite clay (MMT) in preparing intercalated and organic modified MMT, which is then dispersed in diglycidyl ether of bisphenol-A (such as epoxy resin Shell Epon 828) to form a epoxy polymer-clay composite material in a nanoscale dispersion. By using such intercalating agents, the interlayer distances of MMT can be enlarged to 18.0 Å. The epoxy resin can then enter into the interlayer and form an epoxy/clay material through curing polymerization at 75° C. This reference also indicates an improvement in heat distortion temperature. The intercalating agent performs a role of monolayer to bilayer, and even to pseudo-trilayer. The interlayer distance ranges between 13.8–18.0 Å, which allow the epoxy resin to polymerize therein, and further to exfoliate the layered inorganic matter so that performance advantages are achieved.
Japanese Patent No. 8-22946 (Toyota Company) discloses the first commercial inorganic/organic polymer composite material under a nanoscale regime. This composite material is produced by dispersing [H3N+(CH2)11COO−]-montmorillonite in Nylon 6, wherein the aminocarboxylic acid is provided as an intercalating agent and the polymers are formed between layers of the aminoacid intercalated clay through condensation of caprolactam monomers to Nylon 6 polymer. In this invention, since the aminocarboxylic acid intercalating agent is hydrophilic, the modified clay is suitable for Nylon 6 compatibility but can not easily mix with nonpolar polymers such as polyethylene and polypropylene in a uniform manner. Accordingly, Japanese Patent Publication No. 8-53572 provides other organic onium ions as an intercalating agent to mix with layered silicate, which can be uniformly dispersed in molten polyolefin resin. Unfortunately, the organic onium ions can only enlarge the interlayer distances to a certain degree and the affinity between the intercalating agent and the polyolefin resin is too weak to exfoliate the layered silicates.
In general, the difficulty for the exfoliation by using conventional quaternary ammonium salt is caused by the inherent chemical structure of the clay. The chemical structure of the common smectite clays such as montmorillonite consists of ionic pairs of ≡Si—O31 anions on the surface and counter metal cations. The surface ionic charge interaction tightly binds the neighboring silicate plates together and maintains the primary stacking structure. The exfoliation of the layered silicate plates hence requires a tremendous force to overcome the inherent ionic bridges. Moreover, the conventional process requires two separate steps. The layered silicates are first ionically exchanged with an intercalating agent such as amino acid, or alkyl ammonium quaternary salt. The intercalated silicates at this stage are embedded with organic salts and the gallery distance is widened to a commonly 18–40 Å. In the presence of organic intercalants, the modified clays become organophilic and are possibly exfoliated. Accordingly, there is a need to ameliorate the process by means of providing appropriate intercalating agents and operation conditions which may exfoliate the silicate clay directly for better compatibility with other polymer materials. In addition, it is even more desired to prepare the exfoliated form of silicate plates which are free of organic polymers, so that the random silicate plates in pure form and free of organic contaminations can be mixed with different target polymers to produce improved properties without encountering the dispersion problem in the process.